OK so i have had some difficulty in delivering an accurate lighting effect on the chairs as depicted in the attachment. Any thoughts or contributions and suggestions gladly received. The attached is a 10cm render with AO.

I am running Sketchup 2015 (15.3.331 64bit) with Lightup v4.6
Am i correct in thinking that since I have a perpetual license I cannot get v5, and therefore cannot upgrade my Sketch Up?

The issue is that these white chairs show black areas when light with a fixture from above and in front.

You could try right-clicking on a chair and selecting LightUp-Vertex Lit from the context menu. If it works, do that to each chair.

Is the chair from the 3DWarehouse? If so, which one? The chairs on 3D Warehouse have no quality control and, though they may look ok in SketchUp, may be badly formed with reversed/overlapping/intersecting faces. If that is the case, it is necessary to either correct the errors (possibly a difficult and tedious task) or try a new chair.

LightUp is a remarkable renderer, but all renderers have trouble with malformed geometry.

So i tried your method which did not fix the issue but make me look more closely at the geometry of the 3D warehouse chair. It had been built rather strangely and I was able to remove some components and it now lights BEAUTIFULLY!

This really helped. Thanks. I built a unit and had the same problem but thought I had built it perfectly. For the round corner unit I drew a quarter circle, closed it with a 90 degree corner and used the push/pull tool to extrude it. I did increase the amount of sides from default to 50 to make it look smoother, so maybe this is where it got complicated.

I applied Vertex Lit as suggested and it fixed every problem with dark areas in my model. Before and after pics below.